1. Field of the Invention
The present invention relates to pilot operated hydraulic valves, and more particularly to such valves that incorporate mechanisms which compensate for variation of a pressure differential across a pilot orifice.
2. Description of the Related Art
A wide variety of machines have moveable components which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally, the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be positioned adjacent the actuator being controlled. This change in technology also facilitates computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a set of proportional solenoid valves such as described in U.S. Pat. No. 5,878,647. Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot valve element that controls the flow of fluid through a pilot passage in a main valve poppet. Opening the pilot passage releases pressure in a control chamber thereby allowing the main valve poppet to move away from a valve seat which motion creates a path between the inlet and outlet of the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another valve component is spring biased to close the valve when electric current is removed from the solenoid coil.
When an operator desires to move a component of the machine, an input device is operated to produce electrical signals that are applied to the set of solenoid valves for the respective hydraulic actuator, for example a cylinder-piston combination associated with that machine component. One solenoid valve is opened to supply pressurized fluid to a cylinder chamber one side of the piston and another solenoid valve opens to allow fluid being forced from an opposite cylinder chamber to drain to a reservoir, or tank. By varying the degree to which the solenoid valves are opened, the rate of flow into the associated cylinder chamber can be varied, thereby moving the piston at proportionally different speeds.
A drawback regarding conventional solenoid operated pilot valves results from effects produced by a pressure differential that develops across the valve. That pressure differential changes with variation of a load force exerted on the machine component that is operated by the hydraulic fluid from the valve. Variation of the load force and pressure of the fluid supplied to the valve affect the pressure differential across the valve. In the closed state, the pressure differential can affect the amount of force required to open the valve and to produce the desired fluid flow rate there through. As a result, variation of the pressure differential affects the magnitude of electrical current needed to operate the valve in the manner requested by the operator.
Therefore, it is desirable that the design of a solenoid operated valve mitigate the effects produced by different pressure differentials across the valve.